FIG. 7 illustrates a conventional forklift 1C. The forklift 1C includes forks 3 for holding a load 2, cylinders 4 for causing the forks 3 to ascend or descend at a speed corresponding to the flow rate of hydraulic oil, a first valve (e.g., electromagnetic proportional control valve) 5 for controlling the flow rate of hydraulic oil, a second valve (e.g., flow regulator valve) 6 for regulating the flow rate of hydraulic oil between the cylinders 4 and the first valve 5 in accordance with cylinder pressure (the weight of the load 2), a control portion 27 for controlling the first valve 5, and a lift lever 8 for starting/stopping the ascending/descending operation of the forks 3.
The cylinders 4 are connected to a hydraulic portion 10 of the forklift 1C via the second valve 6 and the first valve 5, as shown in FIG. 8. The hydraulic portion 10 includes a tank 10A containing the hydraulic oil, a pump 10B for supplying the hydraulic oil in the tank 10A to the first valve 5, a motor 10C for driving the pump 10B, a hydraulic oil supply path, and a hydraulic oil discharge path.
The control portion 27 includes a current calculation portion 27A for calculating a current command value on the basis of an angle of the lift lever 8, and a current supply portion 27B for supplying the first valve 5 with an energizing current in accordance with the current command value. The lever angle is zero when the lift lever 8 is in neutral position. For example, the forks 3 descend when the lever angle is positive, ascend when the lever angle is negative, and stop when the lever angle is zero.
Incidentally, the forklift 1C has a problem in that the load 2 is vertically vibrated when the ascending/descending operation of the forks 3 is started or stopped. A known solution to this problem is an approach to changing the ascending/descending speed of the forks 3 in two stages. This approach cancels out a vibration caused by the first speed change with a vibration caused by the second speed change, and therefore, the load 2 is inhibited from vibrating (e.g., see Patent Document 1).
This will be described below taking as an example the case where the descending operation of the forks 3 is stopped. As shown in FIG. 9(A), at time t0, the angle of the lift lever 8 is X (where X>0), and the forks 3 are descending at a speed corresponding to the lever angle X.
Once the angle of the lift lever 8 changes from X to 0 at time t1, the current calculation portion 27A decreases the current command value in two stages. Assuming that the current command value is B3 mA when the lever angle is X, the current calculation portion 27A decreases the current command value by half from B3 mA to B4 mA over a period from time t1 to time t1′, and further decreases the current command value from B4 mA to 0 mA over a period from time t2 to time t2′ (see FIG. 9(B)).
The current supply portion 27B decreases the energizing current by half from B3 mA to B4 mA over a period from time t1 to time t1′ and further decreases the energizing current from B4 mA to 0 mA over a period from time t2 to time t2′.
At the center of gravity G of the load 2, a first vibration occurs at time t1 at which the descending speed of the forks 3 is changed for the first time, and a second vibration, which is 180° out of phase from the first vibration and has the same amplitude as the first vibration (strictly, a smaller amplitude due to attenuation), occurs at time t2 at which the descending speed of the forks 3 is changed for the second time (see FIG. 9(C)). Accordingly, the first vibration is cancelled out by the second vibration, with the result that the load 2 is inhibited from vibrating.